Certain embodiments of the present invention are directed to circuits. More particularly, some embodiments of the invention provide systems and methods for bleeder control. Merely by way of example, some embodiments of the invention have been applied to light emitting diodes (LEDs). But it would be recognized that the invention has a much broader range of applicability.
With development in the light-emitting diode (LED) lighting market, many LED manufacturers have placed LED lighting products at an important position in market development. LED lighting products often need dimmer technology to provide consumers with a unique visual experience. Since Triode for Alternating Current (TRIAC) dimmers have been widely used in conventional lighting systems such as incandescent lighting systems, the TRIAC dimmers are also increasingly being used in LED lighting systems.
Conventionally, the TRIAC dimmers usually are designed primarily for incandescent lights with pure resistive loads and low luminous efficiency. Such characteristics of incandescent lights often help to meet the requirements of TRIAC dimmers in holding currents. Therefore, the TRIAC dimmers usually are suitable for light dimming when used with incandescent lights.
However, when the TRIAC dimmers are used with more efficient LEDs, it is often difficult to meet the requirements of TRIAC dimmers in holding currents due to the reduced input power needed to achieve equivalent illumination to that of incandescent lights. Therefore, conventional LED lighting systems often utilize bleeder units to provide compensation in order to satisfy the requirements of TRIAC dimmers in holding currents.
FIG. 1 is an exemplary circuit diagram showing a conventional LED lighting system using a TRIAC dimmer. As shown in FIG. 1, the main control unit of the LED lighting system 100 includes a constant current (CC) unit U1 and a bleeder unit U2. The constant current unit U1 controls constant current output of the LED lighting system 100. The bleeder unit U2 provides a bleeder current of a certain magnitude to maintain the TRIAC dimmer's normal operation and to prevent the TRIAC dimmer from malfunctioning due to insufficient current supply.
The operation process of the LED lighting system 100 as shown in FIG. 1 is as follows: after the system 100 is powered on, an AC input voltage (e.g., VAC) is received by the TRIAC dimmer and rectified by a full-wave rectifier BD1 to generate a rectified voltage (e.g., VIN); the constant current unit U1 generates a constant current for the LED lighting system 100, and the constant current flows through the LED into the constant current unit U1; using the TRIAC dimmer, the rectified voltage (e.g., VIN) received by the anode of the LED usually has a waveform of an AC signal that has been clipped and rectified. As an example, when the waveform of the AC signal is clipped by the TRIAC dimmer or when the rectified voltage (e.g., VIN) is relatively small in magnitude within an AC cycle, the LED does not conduct current because of the insufficient voltage and does not have a current that flows through, causing the TRIAC dimmer to malfunction. Therefore, the bleeder unit U2 often is needed to generate a sufficient bleeder current in the LED lighting system 100 to maintain the TRIAC dimmer in normal operation.
From the perspective of system power, the input power of the LED lighting system 100 includes mainly the LED power and the bleeder power:Pin=Pled+Pbleeder  (Equation 1)where Pin represents the input power of the system 100, Pled represents the power consumed by the LED, and Pbleeder represents the power consumed by the bleeder unit U2.
Hence it is highly desirable to improve the techniques related to LED lighting systems.